Duration timers are widely used to control all types of appliances and are commonly found in the home where they form an integral part of appliances such as microwave ovens, conventional ovens, and room heaters. These timers in the main are of two distinct types, namely "duration of use" timers and "alerting" timers.
(1) "Duration of use" timers are designed to turn off an appliance at the end of a set or selected period of time and are found in appliances such as domestic microwave ovens and room heaters.
The end of the timing period is sometimes signaled by an audible and/or visual device to alert the user of the completion of the timing sequence, the audible and/or visual signal being one of short duration.
The difficulty associated with "duration of use" timers is that it may be some time before the user realizes that the appliance has been turned off as would be the case if the user was distracted by a caller at the door, an incoming telephone call or by any other task at hand.
Therefore if the appliance is required to be used for a period of time longer than originally intended to achieve a desired result, the process interrupted for a considerable period of time before it is restarted. As in some processes, cooking included, sometimes cannot yield the desired outcome if interrupted for a long period of time, it is realized that "duration of use" timers can have considerable limitations in their applications.
It is to be noted that the audible and/or visual alert signal associated with "duration of use" timers is normally of short duration and therefore can very easily result in the signal being missed altogether if the user is not in the immediate vicinity of the appliance when the time times out when the short duration alert signal is produced.
(2) "Alerting" timers are designed to alert the user of an appliance at the end of a selected period of time and are normally found in appliances such as conventional ovens. The audible and/or visual signal is provided as an alert only and the appliance remains operational until manually turned off.
The disadvantage of "Alerting" timers is that they do not turn appliances off so that when appliances incorporating such timers are used or operated by an aged person or by a person who is forgetful by nature, potentially dangerous situations may arise. A typical example is a conventional oven which when left unattended for some time as would be the case if, for example, the operator is distracted by an incoming telephone call, can cause a house fire.